Beta-arrestin2-knockout (Beta-arr2-KO) mice display enhanced morphine-induced biological effects such as analgesia, while other responses like constipation are attenuated when compared to their wild-type (WT) littermates. Further, physical dependence appears to be unaffected by the loss of Beta-arr2 with chronic morphine treatment. While Beta-arr2 ablation produces enhanced morphine-induced analgesic responses, other opiates including methadone and fentanyl do not. We are now asking whether opiate-induced constipation and physical dependence will vary with the opiate drug used. Therefore, the overall goal of the research plan is to test the hypothesis that ablation of Beta-arr2 may alter the development of opioid-induced constipation and physical dependence in an agonist dependent manner. Beta-arr2-KO mice and their WT littermates will be tested in physiological and behavioral paradigms that measure gastrointestinal function and physical dependence after treatment with different opiate agonists including methadone and fentanyl. In addition, in vivo findings will be correlated with ex vivo physiological and biochemical assessments of mu-OR expression and function in these animals. The overall goal of this proposal as a whole is to develop my independent research career by introducing multifaceted approaches to study drug abuse problems in vivo. [unreadable] [unreadable] [unreadable]